What filler do I need for extreme loss of facial fat? (photos)

Although there is no one absolute best choice for you, there are a few good options.  As you are in your 20's, a long term improvement would be advantageous.  Your own fat offers the best chance at permanent replacement fullness.  This is a surgical procedure, either performed in the office under local anesthesia or in a surgery center under IV sedation, with the fat being harvested from an area such as the abdomen or thighs, refined, and then injected into the appropriate facial areas.  Since only about 70% of transplanted fat survives on average, patients often end up having a second treatment in the future.  The fat that does survive will be permanent, as it is your own tissue.  Interestingly, if you gain a lot of weight in the future, that fat may visibly increase in size!  On the other hand, the "off the shelf" fillers such as Voluma or Radiesse seem to last between 1 and 2 years.  Sculptra can last up to 2 years with a touch up along the way, and is a different type of filler in that it stimulates your body to produce its own collagen, thus thickening the skin and soft tissue.  Depending on the exact area of the face and the extent of atrophy, I might choose from any of the above, but keep in mind that only fat has the potential to be permanent.

Sculptra is a great option as it builds collagen which will last about two years. The treatment is generally done in 2-3 sessions spaced 6-8 weeks apart until the result is achieved. From there, you can maintain with yearly treatments. Each treatment is around $3500.Fat transfer is also an option which would be a little more permanent. Liposuction is performed to the abdomen or thighs, and the fat is injected into the face. This is usually done in the operating room, but it can sometimes be tolerated under local anesthesia in the office. Generally 50-70% of the fat survives, so some people desire a second treatment a few months later to enhance the result. The cost is around $3000-5000. Voluma is a new filler from the makers of Juvederm, and it can be used in those areas. It is around $800-1000 a syringe (you'd probably need 2-4), and it lasts up to 2 years. Best of luck achieving the result you want.

Typically for loss of volume in the submalar region Scuptra is a good choice.  You can also use fat grafting.  Both methods will take more than one treatment to achieve significant results.